There are four general conventional methods for sinking large diameter vertical openings. The first method involves complete destruction of the face. This method requires a massive bore hole probe with roller cutters which cover the entire surface area of the opening. The probe that powers such bits is massive and requires great power. This method is suitable primarily for soft rock or medium hard rock, such as up to 4 points of Protodiakonov scale. The drilling speed is very low in hard rock, the roller-cutters wear out quickly, and the method consequently entails high costs and low effectiveness for drilling in hard rock.
Second is the core drilling method. This method employs roller cutters on the periphery of a cylindrical probe, so that approximately 25-30 percent of the surface area of the opening is cut, primarily on the periphery. The main bulk of the rock is drawn to the surface as a core sample or block. The core sample is separated from the underlying rock (undercut) using convention methods such as shaped-charge shells, cable loops, tightening of the tackle-block system and other methods. This method utilizes core drills as bore hole probes and is useful primarily in hard rock and medium hard rock (up to 12 points of Protodiakonov scale).
The third method for sinking vertical openings of large diameter consists of gradual shaft underreaming. This technique is also known as phase drilling. The shaft is widened gradually using a drill of larger diameter in each phase. Conventional such underreamers have employed, for instance, intermediate phases of 3 m, 5.75 m, 7.5 m, and 8.75 m, for a final diameter of 8.75 m. The units utilized for underreaming may be thought of as a variation of the first method, and they thus share its disadvantages including low drilling speed, early wear of the cutters, high cost of drilling, and low effectiveness.
It is also known to employ a fourth technique for sinking large diameter holes. This technique uses roller-cutters which move inwardly in a plane perpendicular to the axis of the shaft. A conventional core drill may, for instance, be adapted to include levers and roller cutters. The drill itself may be a hollow cylinder with a lid in the upper part. The lower part of the cylinder may be a ring which features roller cutters and levers for core sample undercutting.
None of these techniques, however, permit underreaming of shafts in one phase or allow creation of a larger diameter hole through a small bore hole.